The present invention relates to a vehicle suspension device and, more particularly, to a vehicle suspension device having a coil spring, at least a part of which is coated with a synthetic resin.
A vehicle suspension device having a coil spring is mounted to a vehicle, generally with the coil spring exposed to the exterior. Therefore, the coil spring may corrode due to mud splashed during travel, exposure to rainwater or the like.
To avoid corrosion, it has been proposed that a spring wire be coated with a synthetic resin and then be coiled, thus forming a coil spring. The adjacent turns of such coil spring, however, bang together when the spring undergoes surging vibrations caused by impact applied on it while the vechicle is travelling at a high speed. Banging of the turns tears the resin coating, particularly at the end portions of the spring. Thus, the resin coating cannot successfully protect the spring from corrosion. To prevent corrosion of the coil spring it would be necessary to prevent surging vibrations.
To prevent surging vibrations, it has been proposed that the coil spring be embedded in a cylinder of elastic rubber. Indeed the cylinder may suppress surging vibrations and may also protect the spring from water. But, when the cylinder is compressed, the friction between the elastic rubber and each turn of the coil spring increases so much that the coil-cylinder unit comes to have a large spring constant and inevitably has its spring characteristics degraded.